The present invention relates to the formation of hollow molded components, particularly by means of an apparatus in which uncured thermosetting material is laid up about one or more meltable mandrels formed from eutectic salts.
Destructable molds made from low melting point, or eutectic, salts are known in the art. For example, U.S. Pat. No. 1,523,519, issued to Gibbons, addresses the suitability of various combinations of salts for use as mold forms for rubber tires and other vulcanized articles. U.S. Pat. No. 1,554,697, issued to Alden, teaches the use of salt cores in the manufacture of hollow articles, which cores may be extracted from the molded article by dissolving the salt with water after completion of the molding process. Other methods of removing the core or interior mold, namely by breaking or melting same are discussed in U.S. Pat. No. 2,217,743, issued to Dreyfus.
Several advantages are realized by the foregoing schemes. Eutectic salt cores or mandrels may readily be cast in rather fine detail, allowing ample flexibility in the design of articles to be molded therefrom. Furthermore, the ease with which new cores may be recast from the melted, broken or dissolved material of previously used cores lends a desirable economy to the overall operation.
With regard to the use of salt cores to form discrete, non-continuous hollow articles, however, one problem which has not been satisfactorily solved to date is the removal of the core from the finished article. Dissolving the core as in Adler is objectionable because it is time consuming. Hollow, frangible cores, as suggested by Dreyfus, may be more readily extracted, but are costlier and more difficult to produce than solid cores. Solid cores may be melted, but must then be drained by gravity. Regardless of what method of extraction is used, recasting has required collecting and repouring the used salt to form new mandrels, thus interrupting the continuity of the process.
An apparatus for shaping and laminating thermoplastic sheets which includes means for continuously recycling the eutectic salt used in the process is shown in U.S. Pat. No. 2,608,720, issued to Meisner. U.S. Pat. No. 4,056,596, issued to Pahl, discloses another continuous scheme wherein tubing is formed about a hollow mandrel which may be melted by an induction coil or broken up by ultrasonic waves and forced back through the mandrel by compressed air. Neither Meissner nor Pahl, however, is adaptable to recycling mandrels used in forming discrete, non-continuous hollow articles, their utility being limited respectively to the formation of continuous sheets and tubes.
A further objection to present techniques is the lack of control over the compression of the material to be molded. The formation of high strength laminates having a thermosetting resin matrix depends on the uniform exertion of a precisely controlled pressure during the molding or curing process. In the prior art, layers of thermosetting material wrapped about a salt core and placed in a rigid outer die for curing were compressed almost exclusively by the expansion of the salt core due to heating to cure temperature. If instead of a rigid outer die, the uncured component is enclosed in a vacuum bag which is in turn placed in an autoclave, the surfaces exposed to the vacuum bag will usually be compacted sufficiently by the autoclave pressure to be free of voids due to air bubbles, but the inner surfaces must still rely on thermal expansion of the mandrels. Because the manufacturing tolerances of the mandrels, the thickness variation of the uncured material, and the manufacturing tolerance of the outer die, the final fit of the uncured component into the outer die will be difficult to control. This dimensional variation will have a direct effect on the pressure exerted by the thermally expanded mandrels on the component at cure temperature, which, in turn, will determine the strength and quality of a part.
U.S. Pat. No. 2,739,350, issued to Lampman, discloses a reusable, thermoplastic inner mold which may be easily withdrawn from the molded article after completion of the molding process but before the mold cools to a rigid state. The mold includes an inner cavity which is pressurized during the molding operation, resulting in compression of the material to be molded against a rigid outer die. Still, however, a precisely controllable, uniform pressure cannot be applied using the Lampman inner mold because the pneumatic pressure must be transmitted through a rather thick layer of viscous thermoplastic. Another objection to the Lampman method is that the opening to the interior cavity of the article to be molded must be at least nearly as wide as the cavity itself. Furthermore, the cavity must not include any sharp recesses transverse to the direction of removal of the inner mold, thus limiting the shapes which may be formed by the Lampman process.